1. Field of the Invention
The field of this invention relates to orthodontic devices and more particularly to a coil spring which is to be used in an extraoral orthodontic appliance where the coil spring is constructed so as to impart a substantially constant force over the entire range of deflection of the spring.
2. Description of the Prior Art
Orthodontics is a specialty of dentistry which is concerned with the treatment of malpositioned teeth and the correction of improper relationships of the teeth and dental arches. It is common to utilize intraoral orthodontic appliances and extraoral orthodontic appliances to correct improperly positioned teeth. It is also common in both intraoral and extraoral appliances to use some form of spring mechanism so that a force can be applied to the teeth.
In the past, the force producing means for intraoral and extraoral orthodontic appliances has been coil springs, rubberbands and elasticized cloth. Rubberbands lose their force within 24 hours and must be replaced frequently. The patients are required to replace the rubberbands every day and may chose not to replace such a rubberband especially if the rubberband utilizes a higher force and therefore be more painful to the user. The result is when using rubberbands, treatment frequently progresses less efficiently than what it should be. The advantage of rubberbands is that they do impart a substantially constant force onto the teeth once the rubberband is deflected. Increase or decrease of deflection of the rubberband does not significantly alter the application force.
Elasticized cloth straps also have a constant force versus extension characteristics. However, such elasticized cloth straps also need to be frequently replaced, usually within two to three weeks of usage.
The most common type of force producing device is the coil spring. Coil springs that have been commonly used in the past are stainless steel springs. The advantage of springs as a force producing device is that they almost never require replacement during the entire length of time of usage. The disadvantage of most springs that are in common use is that they do not apply a constant force over different deflections. Deflection increases and decreases when the user talks or moves his or her head. Therefore, when using the conventional stainless steel type of spring, the force that is applied is constantly changing. Orthodontists feel that constant force application moves the teeth more efficiently than variable forces. Also variable forces can cause pain especially if the force significantly increases momentarily.
The advantages of using a constant force spring in an orthodontic headgear has been previously known. Reference is to be had to the U.S. Pat. No. 4,849,032 by Kozo Kawaguchi. However, the aforesaid patent has dealt only with the inclusion of constant force springs within an intraoral appliance. When dealing intraorally the springs are quite small in size and therefore of a substantially less force than what would be used in an extraoral appliance. Therefore, the problems encountered in designing such springs for intraoral orthodontic appliances are substantially different than what are encountered in designing constant force springs in an extraoral orthodontic appliance. There is a U.S. Pat. No. 4,264,302 by Roger Wolk et al. which is directed to an extraoral orthodontic appliance which uses a constant force flat spring assembly. The spring of the present invention can be manufactured more economically than the flat spring assembly of Wolk et al.